Nutrients and light limit biomass growth of N2-fixing but not non-fixing trees in tropical forests

Background/Question/Methods:

Tropical forests contribute a major fraction to the land C sink but the role of soil nutrients in limiting tree biomass growth in response to rising atmospheric CO₂ is poorly known.  Recent findings suggest that, following disturbance, successionally young forests may be deficient in nitrogen (N) and/or phosphorus (P), however nutrient manipulations of mature forests have revealed surprisingly weak effects of nutrients on the stem growth of mature individual trees.  It is unclear how such weak experimental nutrient effects are reconciled with the existence of broad geographical correlations between soil nutrients and forest biomass growth.  While tree growth is a complex function of nutrients, light, and canopy status, it is plausible that responses differ across different plant functional types.  Here we use data from the longest running tropical fertilization experiment to ask first whether different functional groups have different nutrient needs, second, whether a differential nutrient limitation response will affect biomass accretion, and third, whether there is an interactive light-nutrient effect. We also used fertilization experiments across three scales (greenhouse, understory, and whole forest) to examine the interaction of light and nutrients.

Results/Conclusions:

We show that, in an intact and biodiverse mature tropical forest in Panama, N₂-fixing trees more than double their basal area growth rate when exposed to increased soil P and N over an 11-year experimental period.  In contrast, there was no effect of nutrient treatment on the growth of non-fixing trees. Change in resident fixer basal area responded by adding 225% more basal area when given N and P (p=0.031) while non-fixing trees showed no significant fertilization effect. Experiments at different scales (greenhouse, sub-canopy, and whole-forest) revealed a strong interactive effect of soil nutrients and light on tree growth and production of symbiotic N₂-fixing nodules.  Contrary to the idea that sub-canopy trees may respond most strongly to nutrients, we found the greatest response in mature canopy-level trees with full access to light.  Our findings suggest that N₂-fixing tree species may play a disproportionately important role in governing tropical forest response to changes in soil nutrients and rising atmospheric CO₂.